The subject invention relates to the field of printed document or image (hereinafter “image”) security, and, more particularly, to determination if a copy detection feature in a printed image is “linked” (i.e., associated in a predetermined manner as will be defined below) to coded information in that image to determine whether the printed image is an original or a copy or has been altered.
Advances in the arts of photocopying and digital image scanning and printing have made it increasingly easy to make copies of printed images with such high fidelity that it is difficult to distinguish between an original printed image and a photocopy or scanned-and-printed copy of the original image. These advances have implications in regard to such secure documents or images as postage meter indicia, paper currency, and event and travel tickets. Therefore, it is desirable to provide secure images with printed images that incorporate special features, sometimes referred to as “copy detection features”, wherein copying of the printed image results in changes of the feature in the copy relative to the original image in a manner that can be detected with a degree of reliability and convenience.
FIG. 1 shows a simplified representation of one such image, postage meter indicium 10. Such indicia are printed on mailpieces by postage meters to verify that the appropriate postage has been paid. (Operation of such postage meters is well known and need not be discussed further for an understanding of the subject invention.) Indicium 10 typically includes textual information such as Post Office identification 12, date 14, serial number 16, and postage amount 20. Indicium 10 also includes graphic elements such as logo 22.
Heretofore such elements were printed with physical graphic security features such as special fluorescent inks or very specific resolution so that it was difficult to copy a postage meter indicium. However, more recently, computer based postage meters, which use commercially available digital printing mechanisms have been developed. These meters lack physical graphic security features. Concurrently, postal services such as the USPS have required that postage meter indicia include postal information in machine-readable and machine verifiable form. In indicium 10 this is provided by two-dimensional barcode 24 which carries the postage amount and other postal information, and which is digitally signed in a conventional manner. Typically barcode 24 is provided in accordance with Information Based Indicia (hereinafter “IBI”) standards of the United States Postal Service.
Because barcode 24 typically is the only part of indicium 10 which is automatically checked when a mailpiece is input to a postal service, it effectively is the indicium and, where meters lack security features, may be easily copied; possibly allowing two attacks:
1) An attacker can make multiple copies of indicium 10 without payment.
2) An attacker can print a high denomination indicium, make multiple copies of barcode 24, print multiple low denomination indicia, and carefully cut and paste high denomination barcode copies into low denomination indicia.
Protection against the first attack can be provided by incorporation of a watermark, as described in the above mentioned copending patent application Ser. No. 10/720,664 “Fragile Watermark for Detecting Printed Image Copies” and Ser. No.: 10/720,292 “Detecting Printed Image Copies Using Phase-Space-Encoded Fragile Watermark”, or by use of any other convenient copy detection feature, such as the commercially available Mediasec Copy Detection Pattern (hereinafter CDP SEAL). While the cutting and pasting of barcode copies might be easily detected at a forensic check point (e.g., visual inspection by a postal service worker); it is likely to pass undetected when first input to a postal service and never be subject to further inspection.
Thus it is an object of the subject invention to provide a method and system for printing an image such as a postage meter indicium, or similar image representing value, and for detecting when such an image has been altered.